The present invention relates to a process for the production of esters from alcohols using acetic acid as the acetylating agent and clays as catalysts.
This invention particularly relates to an ecofriendly process for the production of esters from aromatic, aliphatic, xcex1,xcex2-unsaturated, cyclic and heterocyclic alcohols in the range of C1 to C10 using acetic acid as the acetylating reagent and clays as catalysts dispensing the use of expensive acetic anhydride as a reagent and corrosive and toxic sulfuric acid, sulfonic acids or expensive resins as catalysts. This process totally eliminates the disposal of salts formed consequent to the neutralisation of H2SO4 or sulfonic acids and the use of expensive acetic anhydride.
Organic esters belong to a very important class of chemicals having applications as intermediates in the synthesis of fine chemicals, drugs, plasticizers, perfumes, food preservatives, cosmetics, pharmaceuticals, as solvents and chiral auxiliaries. In industry, the esterifications are generally carried out in the presence of hazardous, toxic and corrosive sulfuric acid and its derivatives or expensive ion exchange resins. To optimise the yields of ester formation from equimolar amounts of alcohols and carboxylic acids, 2-15 molar equivalents of the condensing agents such as sulfuric acid, tosylchloride, triofluoroacetic anhydride, polyphosphate ester, dicyclohexyl cabodiimide, graphite etc., are generally employed. Heterogeneous acidic and superacid catalysts have proved to be useful in some reactions because of their activity, selectivity, reusability, non-corrosivity and virtual absence of effluent treatment which is associated with the homogeneous catalysts.
Reference may be made to a patent CS 254, 048 (C1, C07C 69/14), Nov. 15, 1988, wherein the esterification of BuOH with AcOH at reflux temperature of 79 to 80xc2x0 C. in the presence of H2SO4 catalyst, resulted in butylacetate with 89-90% yield. Reference may be made to a patent CN 1,068,520 (C1. B 01 J 23/10), Feb. 03, 1993, wherein the isopentyl alcohol esterification with AcOH at xe2x89xa7140xc2x0 C. and 100-142 hrs resulted in isopentyl acetate with H2SO4 treated pulverised rare earth compounds as catalysts. The drawbacks in the above processes are longer reaction time, higher reaction temperatures and the use of corrosive sulphuric acid which requires the waste disposal consequent to the neutralisation with bases after the completion of the reaction.
The solid acids, solid super acids and cation exchange resins are very effective catalysts for the esterification process. Reference may be made to a patent, U.S. Pat. No. 3,590,073 (C1. 260-476 R, C07 C), Jun. 29, 1971, wherein the esterification of tert-BuOH with AcOH, was carried out by passing through a column of Amberlyst-15 (the sulfuric acid treated cation exchange resin) at the rate of 0.5 ml/min at ambient temperature to give 25% tert-butyl acetate. Reference may be made to an European patent, EP 66,059 (C1. C07 C69/14), Dec. 08, 1982, wherein the alcohols with 2-5 carbon atoms react with acetic acid in verticle reactor to give corresponding esters in presence of strongly acid ion exchangers as catalysts. Reference may be made to a patent Rom. Ro 72,739 (C1. C07 C 69/14) Jul. 27, 1981, wherein the esterification of xcex2-phenyl ethyl alcohol with AcOH to give an ester in presence of cation exchangers. The drawbacks in the above processes are the use of expensive resins as catalysts, applicable to 1-5 carbon atoms consisting substrates, higher reaction temperatures and lower conversions with poor yields.
Reference may be made to another communication in Synthesis. 1978 (12) 929-30, wherein the esterification of primary and secondary alcohols with acetic acid was performed in the presence of Nafion-H for 4 hrs, resulting in moderate yields (40-60%). The drawbacks in the above processes are the moderate yields and the use of expensive resin catalyst. Reference may be made to a communication in J.Org. Chem. 1996, 61, 4560, wherein the esterification of alcohols with carboxylic acids in the presence of p-nitrobezoic anhydride and acylation of alcohols with acid anhydrides using scandium trifluromethane sulfonate (triflate) catalyst resulting  less than 1 to  greater than 95% isolated yields in 0.5 to 5 h. The drawbacks in the above processes are the use of synthetic triflate catalyst and acetic anhydride as an acetylating agent. Reference may be made to another paper in Chem. Comm 1996, 2625 wherein various alcohols were acetylated in the presence of acetic anhydride (acetylating agent) and trimethylsilyl-trifluoromethanesulfonate (triflate) as catalyst. The yields obtained were 55 to 100% in 0.007 to 2 h. The drawbacks in the above processes are the use of synthetic triflate catalyst, acetic anhydride as acetylating agent and lower yields. The last reference may be made to a communication in Ind. Eng. Chem. Res.1994, 33, 2198, wherein the production of phenethyl acetate and cyclohexyl acetate were performed using a variety of solid acid catalysts such as Amberlyst-15, Filtrol-24, sulfonated zirconia, DTPA/silica and DTPA/carbon and acetic acid as the acylating agent. However, with Amberlyst-15 takes 5 h for 95.5% conversion of phenethyl alcohol while DTPA takes 3 h for a conversion of 100%. The drawbacks in the above process are the use of an expensive resins as catalysts and longer reaction time.
Obviously different approaches have been employed both on laboratory and commercial scale to prepare esters, and the traditional homogeneous catalysed reactions are being less favoured owing to the problems of separation and reuse. The present trend is to develop solid acids from cheaply available sources, and especially of clays.
The main object of the present invention is to provide a process for the production of esters from alcohols acetic using acid as the acetylating agent and clays as catalysts which obviates the drawbacks as detailed above.
Another object of the present invention is the use of non-corrosive and cheaply available heterogeneous solid acid catalysts for easy adaptability in a continuous process.
Still another object of the present invention is to provide the process which is applicable to various substrates of aliphatic, acyclic, cyclic, heterocyclic xcex1,⊕-unsaturated and aromatic alcohols such as octanol, decanol, 2-octanol, amylalcohol, isoamylalcohol, 2-pentanol, cinnamyl alcohol, benzyl alcohol, trans-2-hexene-1-ol, cyclohexanol, cyclopentanol, 1-phenyl ethanol, p-methoxy-1-phenyl ethanol, p-methyl-1-phenyl ethanol and furfuryl alcohol.
Still another object of the present invention is to provide a process which takes place at lower reaction temperatures.
Yet another object of the present invention is to provide the process in short reaction time intervals.
Accordingly, the present invention provides a process for the production of esters from alcohols using acetic acid as the acetylating agent and clays as catalysts which comprises the preparation of esters in a single step from the reaction of aliphatic, acyclic, cyclic, heterocyclic, xcex1,⊕-unsaturated and aromatic alcohols having carbon atoms in the range of C1 to C10 with acetic acid in the molar ratios of 1:3 to 11 using reusable natural montmorillonite/metal ion exchanged clay catalysts such as Fe3+, Cu2+, Zn2+, Al3+, Ce3+ or Zr4+-montmorillonites in the solvent medium of aliphatic, aromatic, or chlorinated hydrocarbons or without a solvent for at least at 30-140xc2x0 C. for a period in the range of 0.02 to 3.0 hrs. and recovering the corresponding esters by simple work-up procedure.
In an embodiment, the catalysts were selected from naturally available clays or metal ion exchange clays such as Fe3+, Cu2+, Zn2+, Al3+, Ce3+ or Zr4+-montmorillonites.
In another embodiment, alcohols with carbon atoms in the range of 1-10 may be used and selected from octanol, decanol, 2-octanol, amylalcohol, isoamylalcohol, 2-pentanol, cinnamyl alcohol, benzyl alcohol, trans-2-hexene-1-ol, cyclohexanol, cyclopentanol, 1-phenyl ethanol, p-methoxy-1-phenyl ethanol, p-methyl-1-phenyl ethanol and furfuryl alcohol.
In still another embodiment, the solvents used for the reactions may be selected from aromatic hydrocarbons such as dichloroethane, acetic acid, chlorobenzene and toluene.
Is still another embodiment, the recovery of the corresponding esters was carried out by separating the catalyst through filtration, the excess acetic acid was neutralised by treating with saturated NaHCO3 solution and removing the aromatic hydrocarbons by concentration in rotavapor.
Preferably, the present process for the preparation of esters from alcohols using acetic acid as acetylating agent and clays/modified clays as catalysts in solvent/without a solvent medium (in the presence or absence of a solvent) at 30xc2x0 C. for a period of 0.02 to 2.5 h, and recovering the corresponding esters by easy work-up. The clay catalysts used may be selected from metal ion-exchanged montmorillonites/natural montmorillonite clays.
One of the major advantages involved in the present invention is use of abundantly available montmorillonite from nature as a catalyst for the acetylation of various alcohols with acetic acid (acetylating agent), preferably, without any further purification as shown in (example 1b) for the first time. The activity of the natural montmorillonite is comparable with the Cu2+-montmorillonite (Cu2+-exchanged K10 montmorillonite), which is synthesised from commercial K10 montmorillonite (acid treated montmorillonitexe2x80x94Fluka). K10 montmorillonite as such will lead to too many side products [ethers, olefins(dehydrated product), polymerised products, alkylation on the aromatic solvents used in the reaction] instead of giving the corresponding ester with the alcohol and acetic acid due to its higher acidic strength. Exchange of K10 montmorillonite with metal ions changes the acidic strength of the catalyst which was found to be suitable for the exclusive formation of the desired ester. In industries, the esterifications were generally carried out in presence of the sulfuric acid and resins (ion-exchange) catalysts. The resins are expensive and they loose the activity gradually. In some cases the removal/separation of the catalyst from the product become more problematic due to their rigidity, whereas the cheaply available clay, the natural montmorillonite, overcomes the above problems with consistent activity for several cycles.
Metal ion-exchanged montmorillonite clay catalysts were prepared as described in example 1 and employed them in the esterification of alcohols with Acetic acid (acetylating agent) as described in examples 2 to 21.